The issue is that the thumbnails themselves have a black border. Notice that if you view one of the thumbs directly from YouTube, you'll see the black bars: http://i.ytimg.com/v...M/hqdefault.jpg.

Since all the thumbs appear to have the exact same bars on top and bottom, you could probably come up with some custom CSS that would hide the bars completely. For instance, here's a screenshot of the following CSS modifications:

No problem. I would recommend adding the CSS from my post to the bottom of your WordPress theme's style.css (wp-content/themes/critical/style.css). You can either use WordPress's theme editor or any text editor to do this. Keeping the CSS there will protect it from being wiped out when you upgrade TubePress.

Let me know if you'd like any assistance at any step along the way. Thanks!